The human paramyxoviruses have long been recognized as important respiratory pathogens of children, as well as adults. For example, human parainfluenza virus 3 is a major cause of pneumonia and bronchitis. Paramyxoviruses enter cells and spread between them via membrane fusion. As we learn more about the mechanism of paramyxovirus-induced fusion, it is becoming apparent that its fusion protein is similar in many aspects to those of the influenza and human immunodeficiency viruses. However, the paramyxoviruses are unique among fusion-promoting viruses in one very important aspect: their receptor recognition and fusion-promoting activities are contributed by different spike proteins. This necessitates a mechanism by which the two proteins can transduce the juxtaposition of the two membranes, mediated by the attachment protein spike, into their fusion, mediated by the fusion protein spike. Characterization of the determinants of the virus' interaction with cellular receptors and how this event alters the relationship between the two paramyxovirus spike structures to effect fusion are important aspects of the viral infectious process and the mechanism of disease progression. Completion of these aims will contribute to our long-term objective of understanding the early interactions of the virus and the cell surface, mediated by the viral surface glycoproteins.